The mechanisms of clinical autoimmune disease have proven to be difficult to elucidate. Evidence implicating molecular mimicry, sub-dominant epitopes, immune ignorance, and failure of the apoptotic cellular apparatus as contributing to the mechanisms of autoimmunity have been convincingly demonstrated in various experimental systems. Perhaps, other mechanisms exist. We propose that somatic mutation is a possible mechanism to initiate autoimmunity and further propose to experimentally evaluate an example of autoimmunity to the nuclear antigen La, which is targeted in Sjogren's syndrome and systemic lupus erythematosus. Presumably, the new protein structures created by somatic mutation would have been previously unknown to the immune system. Once the efferent immune response against the new structure(s) extends to non-mutated structures of the antigen, then an autoimmune response would be established. Serendipitously, we detected examples of somatic frame shift mutations in a mutational hot spot of human La in anti-La precipitin-positive patients at both the DNA, RNA and protein level. Recent preliminary data indicate that such mutations may occur in up to 50% of La positive patients, while no mutations were detectable in healthy donors and age-, sex-, and race-matched controls. Insert or deletion in the hot spot leads to a subsequently altered amino acid sequence with truncation of the La antigen. Such mutations could result in novel B- and T-cell epitopes. Indeed, we found antibodies to the hot spot region in autoimmune patients. Furthermore, we created genomic mutant and native La gene constructs and established transgenic animals in which these constructs were permanently expressed. These genomic constructs will be used to create a genomic clone allowing us to develop an animal tolerant to native human La in which we can switch on the expression of the human mutant La form. We herein present an experimental plan to further explore somatic mutation of La and its potential for generating autoimmunity. We have developed highly sensitive and specific assays to detect the somatic mutation at the DNA, RNA and protein level. According to power calculations we will extend our preliminary studies by applying these techniques to anti-La positive and anti-La negative age-, sex- and race-matched patients as well as healthy controls. Furthermore, we will determine whether or not somatic mutation in La can create novel B- and T-cell epitopes in experimental animals and autoimmune patients. Finally, using the Cre loxP system we will generate an animal model imitating the particular somatic mutations observed in man. These experiments should help establish whether somatic mutation is a plausible mechanism for autoimmunity.